Treatment of metal surfaces



nitrate salts.

United States Patent-O ice TREATMENT OF METAL SURFACES Claims priority,application Italy May 8, 1950 8 Claims. (Cl. 148-6.15)

The present invention relates to new and useful compositions in thetreatment of metals such as iron and iron alloys, zinc and cadmium,particularly by coating the metals with a phosphate layer, and to aprocess of treating the metals.

The present invention is a continuation-in-part of my copendingapplication Serial No. 223,166 filed April 26, 1951, now abandoned forProcess for the Preparation of Nitrophosphate Salts or Multi-Acid andMultimetallic Salts for the Nitrophosphatization of Iron, ItsDerivatives and Alloys as Also of Cadmiumand Zinc.

The formation of phosphate coatings on iron, zinc and cadmium substancesby the use of free phosphoric acid and acidic phosphate compounds hasbeen well known. An oxidizing agent is also employed'in conjunction withthe phosphoric acid and acid phosplate compounds for the formation ofthe phosphate coating. The most suitable oxidizing agent was found to benitric acid plus- I-Iowever, compositions containing phosphoric acid,nitric acid, phosphates and nitrates, although the same are effective inthe treatment of the metal have been found to be disadvantageous in thatthe solutions have to be prepared fresh.

The use of nitrates in combination with phosphates for the formation ofphosphate layers .on metals shall herein be referred to asnitrophosphating" and the solutions used for this purpose shall bereferred to as nitrophosphating solutions. Nitrophosphating has becomewell regarded and is used to a much greater extent at the present timethan is simple phosphating because of the much greater speed ofproduction of phosphate layers obtained by this method. For example,whereas simple phosphating requires a treatment time of from 60-90minutes, the time required by the use of a nitrophosphating solution isreduced to from /i-4 minutes.

However, the lack of chemical stability of nitrophosphating solutionshitherto known has been a great disadvantage. The lack of stability isprimarily due to a loss in total acidity which is apparently caused by adis placement of the nitric acid and is more pronounced, the greater theconcentration of nitric acid. Due to the instability of the previouslyknown solutions, it was impossible to obtain consistent efficiency andafter standing a few days it required a great deal more of the solutionto achieve the same eifectiveness as with fresh solutions.

Many attempts have been made to overcome this difficulty. It was firstattempted to overcome this difl'iculty by'the addition of alkalinenitrates to the solution. However, this was unsuccessful. It was thenattempted to overcome the difliculty by the addition of many differentsubstances to the solutions, including glycerin, lanolin,'ethylene oxidederivatives, and thiourea, all of which have proved to be unsuccessfulfor one reason or another.

It is therefore an object of the present invention to provide for theformation of nitrophosphating solutions containing free phosphoric acid,free nitric acid and phosphates and nitrates, which solutions are stableover long periods of time.

the present invention.

2,787,565 Patented Apr. 2, 1957 It is an additional object of thepresent invention to provide stable nitrophosphating solutions whichretain maximum efiiciency without substantial loss of acidity over longperiods of time.

It is a further object of the present invention to provide a process ofproducing stable nitrophosphating solutions.

It is another object of the present invention to provide a process forthe coating of metals with a phosphate layer.

With the above objects in view, the present invention mainly comprisesan aqueous solution of at least one metallic phosphate, at least onemetallic nitrate, phosphoric acid and nitric acid and at least onephosphoprotein. It has been found that the phosphoproteins which are thealbumins derived from milk, eggs and plants in low concentration in thenitrophosphate solution give the desired stability to the solution sothat the same has substantially constant activity over prolonged periodsof time. The amount of the phosphoprotein is preferably less than theamount of the nitric acid and the phosphoric acid.

The present invention which comprises the utilization of milk, egg andplant albumins in combination with solutions for the production ofphosphate layers on metals and particularly ferrous metals is mainlyapplicable to multi-acid and multi-metallic solutions which are used forthe production of phosphate layers and which, as explained above areherein designated as nitrophosphating solutions. These solutions aredesignated as multi-acid because they contain more than one acid, namelyboth phosphoric acid and nitric acid. Preferably the solutions are alsomulti-metallic in that the phosphates and nitrates in the solution areof more than one metal, such as zinc, manganese, copper, nickel, etc.

The nitrophosphating solutions with the phosphoprotein added thereto arepreferably prepared as liquid salt concentrates which are diluted beforeuse with water so that the liquid salt concentrate is between 120% byweight of the final diluted solution. The liquid salt concentratecontains the phosphoprotein in an amount less than the amount of eitherthe free nitric or the free phophoric acid and preferably between 03-25%by weight. Although greater amounts of phosphoprotein may be utilized,it has been found that as little as 0.3% by weight of the phosphoproteinwill have a sufiicient stability action and therefore greater amountsthan 2.5% by weight are unnecessary. The preferred amount ofphosphoprotein is between 0.5-1.0% by weight of the concentrate.

The amount of phosphate in the liquid salt concentrate, including boththe metallic phosphates and the free phosphoric acid, may vary between4075% by weight of the concentrate. The amount of nitrate in the liquidsalt concentrate, including both the metallic nitrates and the freenitric acid, may vary between 20-50% by Weight of the concentrate. Theamount of water in the liquid salt concentrate may vary between 348% byweight of metallic nitrate to free nitric acid may vary within extremelywide limits without in any way departing from It is only essential thatthe solution contain nitric acid partially combined with a metal andpartially in the form of free nitric acid and that it contain phosphoricacid partially combined with a metal and partially in the form of freephosphoric a-cid.t The only limiting feature in this respect is thatpreferably the amount of total acidity of the solution is between 3*?-Although the scope of the present invention is not meant to be limitedby any theory as to how 1 e stabiliu ing action of the phosphoproteinoperates, it is believed that the same is partly due to a bufferingefiect. Neverthelessyit is mainly of interest to note, that thephosphoprotein according to' the present invention maintains aconstantand unvaried ratio between the undissociated and dissociatednitric acid.

The phosphoproteins, namely the milk; egg and plant albumins are wellknown: and may be utilized according t'othe resent invention nomattcrwhat'process'is utilized for the production of the phosphoprotein'.Thefolloirin'g'; however, illustrates a process of obtaining thephosphoproteih milk albumin or laetalbumin from milk. 1

The milk is skimmed and centrifuged. The milk residue thus obtained istreated with' sulfuric acid in" an amount of 430-440 g. sulfuric acidper each 1000 liters of milk, thus forming a precipitate labile calciumphosphoprotein. instead of sulfuric acid it is' possible to utilizehydrochloric acid, however in the ratio of 665-670 g. hydrochloric acidper each 1000 liters of milk;

The obtained precipitate is washed With-water, dried in-hot air at atemperature greater than 47 C., and then pressed and ground to powderform. The resulting product is a white amorphous powder which istasteless and has a density of about 1.25-'l.2 6, is" insoluble inwater, alcohol and ether; soluble in alkaline solutions, partly solublein concentrated-acid solutions and compietely soluble indiluteacidsolutions: Thereforeup'on dilution of the liquid saltconcentrate containing the phosphop'rotcin, a's'above described,and'heatin'gat the hydrolysis temperature, i. 50 90 Czit is easilypossible to completely dissolve the phosphoprotein contained in'theconcentrate.

Egg albumin and plant albuminmay' be prepared by substantially the samemethod above describedfor'the preparation of milkalbumin; and thee'ggalbumin and plant albumin are even more soluble in dilute a'eidsolutions than is the milk albumin. Althou'gh eg'g,plantandni'iik'albumin are all suitable according to the present invention forthe stabilization of nitrophos'phatingsoliitions, the best results havebeen obtained 'with millt'al'bu min. Although no exact chemical formulacan be given for'the milk albumin, it is known to consist of "thefollowing:

Percent by'w eight" Animal fats 0.5-1' Protein substances (i. e. labilecalcium phospho protein) 80-82 Moisture 10-1-2 Ash 5-7 The presentinvention mainly comprises the addition to a concentratednitropho'sphating solution' which" is diluted before use with water inan amount such that the concentrate is' between- 120% by weight oftheffinal' diluted-solution; of'a phosphoprotein namely milk, egg

and plant albumin: in an-amount of. 0.345% by weight of the concentrate.The proportionsofm'efal'li'c -phos phates phosphori'c acid, metallicnitrat'esandnitric acid niay. be varied within-very wide limits'in thesolutionsso long' as the proportions are suitable to givethedesir'ednitrophosphating properties. concerned mainly with the addition of thepliosphopro tein to -the solution: rather than with: the/"specificproperdens ehother components ofi'th'e solutions? r The presentinvention is 4' tl te tltelsss thet lqw nsstamples. eaver; to. i stratepreferred liquid salt concentrates according to the present invention,the scope of said invention not however 'being limited to the specificexamples given.

EXAMPLE I A liqu id salt concentrate prepared containing thefollowingcomponen'ts:

Percent by weight Zn(HaPOXlzTZHgG43121 04 70 ZntfNOaM-l- HNO; 24 Milkalbumin 0.5 Water 5.5

The ratio" of total 'aciditytto free acidity of the above is 3:1. The\veightof'the' free nitric .acid'is about 34% of the weight of the freephosphoric acid.

EXAMPLE ii A liquid salt concentrate is prepared containing thefollowing components in the proportions indicated:

Pe'rcentjby weight Zn (NQaDiFMMNOElz-FHNO? 23 Egg aibuinin's; 1 Water 6The ratio of total acidity tofree'acidity of the above is 75]. and theweight of thefree' nitric acid is about 33% or the f'ree-phosphoricacid.

EXAMPLE In A coneenttate for .nitr'ophosphating solutions ispreparcd'containing the following components in the proportionsindicated:

I i v Percent by Weight ZnHdRO l IMnG U' 68 ZntNQ); (PQ3) 2'-1''Ni( NO3)2F C1''(NQ3')?+HNOQ3 Plant-protein (from soy'abe'ans) 013 Water 4.7

Theratio of total aciditytofree acidity of the above is 4:1 and the,weight of'f -reenitric acid is about 40% of the weightof freephosphoricacid.

EXAMPLE iv A concentrate: for nitrophosphating solutions is preparedcontaining the following components in the proportions indicated:

Percent by weight angst to free acidity of the above is 5.511; and theweight-ofthe free nitricacid is about equal to the weight ofgtheifr'eephosphoric acid The following is-illustrat'ive of tiieprefe'rre'clprocess of preparing a-liquid' salt concentrate according to the presentinvention 7 Into an acid; resistant vessel is placed concentrated-nitric-facid and concentrated phosphoric acid in apredetermined-ratioas'in any-of the above examples, for example innratio of-70 parts phosphoric acid to 24 parts nitric acid-asin EtampleL- and the acids are'b'eate'd to a temperature of 70 89? and apredetermined-quantity of the phosph'opr'otein, suchasmilk, egg-or plantalbumin is added,--i. e. 0.5 part byweight-of the final concentrate asin Example-I. Then the particular metals such as zinc, manganese,nickel= or' the like" are added in a quantity SuchEthatth'eratiocf totalacidity to free acidity of the r final fconcent'rate'zis between 3-7.

In Example I the ratio of total acidity to free acidity is adjusted to3:1"

The liquid salt concentrates of the present invention generally have adensity which varies between 1.45-1.6 and have in suspension a reddishsubstance of colloidal consistency. The particles forming the suspensionare the albumin. Upon dilution, these particles are dissolved and theclear solutions formed. The product is sold commercially in the form ofthe liquid salt concentrate which is diluted by the user in an amountsuch that the final solution contains the liquid salt concentrate in anamount of l20% by weight.

The liquid salt concentrate is preferably diluted in the proportion of7-15% concentrate to 93-85% water. The solutions are used for thetreatment of metals in an amount of about 1000 cc. per each square meterof surface to be nitrophosphated, requiring about eight hours of workingtime. It is not sufficient industrially if the solution remains in goodcondition for 1-2 days, but it is indispensable that the solution remaineffective for the production of phosphate layers on the metal inaccordance with a standard for 30 or more consecutive or alternate workdays. This was impossible prior to the present invention. However,according to the present invention the solutions remain effective overextended periods of time and even for 1000 days or more.

There are various methods of measuring the efliciency ofnitrophosphating solutions which allow for the determination of theeificiency of the stabilizing action of the phosphoprotein added to thenitrophosphating solutions according to the present invention. Onemethod of measurement is based upon the measurement of the total acidityof the solution when fresh and after a certain time period. According tothis method the total acidity of the diluted solution is determined by astandard acidimetric measurement in accordance with which the solutionis titrated with 0.1 N NaOH using phenolphthalein as an indicator. Forexample, cc. of the diluted solution are taken and 3 drops ofphenolphthalien are added thereto. 0.1 N NaOH is then added until thecolor change of the phenolphthalien indicator. The number of cc. of NaOHrequired corresponds to the number of cc. of total acidity of thesolution and is expressed as points of acidity. So, for example solutionI diluted in a ratio of 7% concentrate to 93% water will have 45-46points of total acidity and the same solution diluted in a ratio ofconcentrate to 85% water will have a total acidity of 92-94 points.

The following tests illustrate the stabilizing action of thephosphoproteins according to the present invention.

Test A A concentrate is prepared according to any one of Examples I, II,III or IV without the addition of the phosphoprotein thereto. Theconcentrate is diluted with water to form a 10% solution. The totalacidity immediately after preparation is taken as 100%. After 48 hoursit is found that the total acidity of the solution is between 92-95% ofthe original total acidity. After 10 days, heating the solution at 80 C.for 8 hours a day (a total of 80 hours of heating) the remaining totalacidity is found to be 75-78% of the initial. The loss in total acidityis apparently due to the elimination of nitric acid.

Test B A concentrate is prepared as in Test A with the addition of aphosphoprotein such as milk, egg or plant albumin in an amount of 0.5%by weight of the concentrate. The'concentrate is diluted with water toform a 10% solution. The total acidity immediately after preparation isthe same as in Test A and is taken as 100%. After 48 hours the solutionis found to have no loss of total acidity, the total acidity still beingat 100%. After 10 days, heating the solution at 80 C. for 8 hours a day(a total of 80 hours heating) the remaining total acid-ity'is found tobe between 95-97% of the initial total acidity.

greases Here too, the loss of total acidity is due to the elimination ofnitric acid. However, as may be seen, the phosphoprotein achievesa'profound stabilization of the solution.

Another method of determining the efficiency'of the nitrophosphatingsolution is based upon the number of square meters of metal that iscoated by a specified quan tity of the solution.

In order for the phosphate layer on the surface of the metal resultingfrom treatments with the nitrophosphating solution to be satisfactory,it must meet certain requisites, as for example:

1. The thickness of the layer must be between 3-10 microns, the thinnerthe better;

2. The resistance to corrosion must be at least of the resistanceachieved by the same layer'produced by a freshly prepared solution,namely on the first day of preparation.

3. The amount of iron must not be above a certain limit, and for exampleit should not exceed 1-2.5 g. per- 1000 cc. of solution.

The more closely the solutions correspond to requisites 1, 2 and 3mentioned above, the better the quality of the phosphate layer produced.It is an accepted principle in this field that a freshly preparednitrophosphate solution has the property of generating nitrophosphatelayers of optimum quality but that the quality becomes progressivelyworse as the solution becomes older and is 75-80% of the initialefficiency after a few days and is only 20-25% of the initialefii-ciency after 1000 days. The progressive lessening of efficiencybecomes more pronounced after the second day of use of the solution andis probably due to a series of secondary chemical reactions to which thesolution is subjected, for example the following: v

a. Variation of the oxidimetric state which generally is constantlyreduced as time passes. This is generally referred to as the RedOxpotential;

b. Variation of the initial pH curve, the initial pH which may forexample be between 2.1-3 depending upon the salt used and upon therelationship between the quantity of acid and the quantity of metals.The value fluctuates as the solution is used due to changes-in the pHand the relationship between the nitric acid and the phosphoric acid aswell as between free acidity and total acidity and the increase of ironcontent in the: solution.

The addition of a phosphoprotein to the solution according to thepresent invention stabilizes both a and b above so that the solutionretains its effectiveness for 1000 days or more and even after 1000 daysthe solution still maintains 80-85% of its initial effectiveness.

An additional means of determining the efiiciency-of thenitrophosphating solution which clearly establishes the stabilizingeffect of the addition of the phosphoprotein to the solution is asfollows: 1

Test C A salt concentrate is prepared according to any one of Examples1, II, III and IV without any addition of phosphoprotein. Theconcentrate is diluted'with water the initial value. After the 10 days,the following checking determinations are made:

(1) The consumption of salt expressed as 'g./sq. m. is determined andfor this solution is given an arbitrary value of 100.

(2) The protective value of the nitrophosphating layer obtained by thissolution is determined from the 5th day on and for this solution isgiven the arbitrary value of This solution is used for the nitrophos-iarses-as A salt concentrate isyprepared as in TestC however with theaddition of 1% of milk albumin. The concentrate is diluted in water inan amount such that the concentrate is of the final solution. Thissolution therefore the same as in Test C abovetexcept for thephosphoprotein content. This solution is used for the nitrophosphatingof 1 square meter of a metallic surface for each 1000 cc. of solutionfor 8 hours a day during 10 days and with daily replenishment of theconsumed salt to theinitial value. The following checkingdeterminationsare made and compared to the values obtained utilizing the solutionsofTest C2,

(.1) The consumption of saltexpressed-es. g./sq..m. undetermined andcompared with that of Test C: above. Compared to the arbitrary value ofTest C the amount ofsaltis 60. It .is therefore aparent that by theaddition of the phosphoprotein, .thejsame, amount of metal may betreated utilizing only slightly more than'one half the amount required.without the. phosphoprotein.

(2) T he protective value of the layeiaobtained, by this solution isdetermined from the 5th ,day on andcompared to that obtained in. Test C..The value isfouhd to be 250 as compared to the arbitrary value of '100from Test C. It may therefore be seen that .the protective layerobtained by the addition of phosphoprotein to the solution is about 2 /2times that obtained without the phosphoprotein.

,While theinvention has been illustrated. and described as embodied intreatment ofmetals,.itis. not intended to be limited to the detailsshown, since various niodifica tions and structural changes maybe madewithout departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention. that others can by applying current knowledgereadily adapt it for various applications without omittin'gifeaturesthat, from the standpoint of prior art, fairly constituteessentialcharacteristics of the generic or specific aspects of thisinvention-and, therefore, such adaptations should .and are intended tobe comprehended within-the meaningand range of equivalence of thefollowingclaims.

What is claimed as new and desired to be secured by Letters Patent is:

l. A liquid salt composition adapted tobe diluted: with water in anamount such that said composition is between 1-20% by weight of thediluted solution which is adapted to be used for the treatment ofmetallic surfaces, said liquid salt composition essentially consistingofat-least one metallic phosphate, at least one metallic nitrate, freephosphoric acid, free nitric acid and at least onephosphoprotein, saidphosphoprotein being present in an amount less than the amount of saidfree acids and at least in an amount of 0.3% by Weight of said liquidsalt composition, the totalacidity of said liquid salt composition beingbetween 3-7 times the free acidity thereof.

2. A liquid s'alt'compositiorl adapted tobe'diluted' with water in ananiountfs'uch' that said composition, is be twee'r'i" 140% by weight ofthediluted solution which is adapted to be used for" the treatmentofm'etallic. surfaces, said liquid salt composition essentiallyconsisting of jat least one metallic phosphate, at least one metallicnitrate, free, phosphoric acid, free nitric-acidaa'nd at least ofn'ephosph'op'rotein, said phosphoprotein being present in an amount between0.3-2.5 by ,weight of said liquid salt composition, the total acidityofsaidliquid saltcomposition being between 3-7 times the free aciditythereof.

3. A liquid salt composition adapted to be diluted with water in anamount such that said composition is between l% by weight of the dilutedsolution which is adapted to be'used for the treatment of metallicsurfaces, said liquid salt composition essentially consisting of3=l8water by weight, at least one metallic phosphate, at least 8 onemastitis-aerate; fiee' phosphoric acid, free nitric acid and at least'one phosphop'rotein,:said phosphoprotein being present-in an amountbetween 0.3-2.5 by weight of said liquid salt composition, the total;acidity of said liquid salt composition being between 3-7 times the freeacidity thereof. w g

4. A liquid salt composition adapted to be diluted with water in anamount such that said composition is between l20% by weight of thediluted solution which is adapted to be used for the treatmentofmetallic surfaces, said liquid salt compositioness entially consistingof 3-18% water by weight, at least one metallic phosphate, at least onemetallic nitrate, free phosphoric acid, free nitric acid and at leastone phosphoprot ein, said phosphoprotein being present'in an amountbetween 05-10% by weight of said liquid salt composition, the totalacidity of said liquid salt composition being between 3-7 times the freeacidity thereof. V

5. A liquid salt composition adapted to be diluted with water in anamount such that said composition is between l-20% by weight of thediluted solution which is adapted to be used for the treatment ofmetallicsurfaces, said liquid salt composition essentially consisting ofat least one metallic phosphate, at least one metallic nitrate, freephosphoric acid," free nitric acid and at least one phosphoproteinselected from the group consistingof milk, egg and plant albumins, saidphosphoprotein being present in an amount less than the amount of saidfree acids and at least in an amount of 0.3% by weight of said liquidsalt composition, the total acidity of said liquid salt compositionbeing between 3-7 timesthe free acidity thereof.

6. A liquid salt composition adapted to be diluted with water inanamount such that said composition is between l-20% by weight of thediluted solution which is adapted to be used for the treatment ofmetallic surfaces, said liquid salt composition essentially consistingof at least one metallic phosphate,- at least one metallic nitrate, freephosphoric acid, free nitric acid and at least one phosphoproteinselected from the group consisting of milk, egg

and plant albumins, saidphosphoprotein being'present in an amountbetween 0.3-'2.5% by weight of said liquid salt composition, the totalacidity of said liquidsalt composition being between 3-7 times the freeacidity thereof.

7. A liquid salt composition adapted to be diluted with water in anamount such that said composition is between l-20% by weight of thediluted solution which is adapted to be used for the treatment ofmetallic surfaces, said liquid salt composition essentially consistingof 31-8% water by weight, at least one metallic phosphate, at least onemetallic nitrate, free phosphoric acid, free nitric acid andat least onephosphoprotein selected from the group consisting of milk, egg and-plantalbumins, said phosphoprotein being present in an amount between 03-25%by weight of said liquid salt composition, the total acidity of saidliquid salt composition being the free acidity thereof. i

8. A process of producing a liquid salt composition adapted to be dil uted' with water in an amount such that between 3-7 times said compositionisgbetween 1-20% .by. weight of the diluted solution which is adapted tobe used for the treatment of metallic surfaces, comprising the steps ofmixing concentrated nitric acid with concentrated phosphoric acid in anamount such that said nitric acid is between 3/10 to equal the amount ofsaid phosphoric acid; adding" apowde red phosphop'rotein to the thusformed mixture in a predetermined quantity adjusted to be between 03-25%by weight of the final composition; and dissolving inj'said mixturehaving said phosphoprotein added thereto atleastone metal selected fromthe group consisting of zinc, manganese, nickel and copper in an amountsuch that the. total acidity of the resulting mixture is between 3 7times the free acidity thereof.

(References on '1 following page) References Cited in the file of thispatent UNITED STATES PATENTS Williams Nov. 4, 1924 Gravell Aug. 11, 19255 Wallace Feb. 3, 1931 10 Darsey Aug. 25, 1942 Tanner et a1. Nov. 17,1942 McBride Mar. 22, 1949 Shawcross Apr. 12, 1949

1. A LIQUID SALT COMPOSITION ADAPTED TO BE DILUTED WITH WATER IN ANAMOUNT SUCH THAT SAID COMPOSITION IS BETWEEN 1-20% BY WEIGHT OF THEDILUTED SOLUTION WHICH IS ADAPTED TO BE USED FOR THE TREATMENT OFMETALLIC SURFACES, SAID LIQUID SALT COMPOSITION ESSENTIALLY CONSISTINGOF AT LEAST ONE METALLIC PHOSPHATE, AT LEAST ONE METALLIC NITRATE, FREEPHOSPHORIC ACCID, FREE NITRIC ACID AND AT LEAST ONE PHOSPHOPROTEIN, SAIDPHOSPHORPROTEIN BEING PRESENT IN AN AMOUNT LESS THAN THE AMOUNT OF SAIDFREE ACIDS AND AT LEAST IN AN AMOUNT OF 0.3% BY WEIGHT OF SAID LIQUIDSALT COMPOSITION, THE TOTAL ACIDITY OF SAID LIQUID SALT COMPOSITIONBEING BETWEEN 3-7 TIMES THE FREE ACIDITY THEREOF.